The Architecture of an OS Monolithic Operating Systems Structure - - PowerPoint PPT Presentation

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Operating Systems Structure and Processes Otto J. Anshus University of Tromsø/Oslo

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The Architecture of an OS

• Monolithic
• Layered
• Virtual Machine, Library, Exokernel
• Micro kernel and Client/Server
• Hybrids

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Goals of the architecture

• OS as Resource Manager
• OS as Virtual Machine (abstractions)
• Efficiency, flexibility, size, security, … as

discussed earlier

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User process

C a l l a s e r v i c e i n O S

Services

Data from network Interrupt handler:

Interrupt Hardware

Operating System Kernel

Service Service Service Service

Start requested service Start (next?) user program

Overhead

• UL -> KL
• UL address space -> UL addr. space

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Monolithic

• All kernel routines are

together

• A system call interface
• Examples:

– Linux – Most Unix OS – NT (hybrid) Kernel many many things entry User program User program call return

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Layered Structure

• Hiding information at

each layer

• Develop a layer at a time
• Examples

– THE (6 layers, semaphores, Dijkstra 1968) – MS-DOS (4 layers) Hardware Level 1 Level 2 Level N . . .

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Microkernel and Client/Server

• Micro-kernel is “micro”
• Services are implemented

as user level processes

• Micro-kernel get services
• n behalf of users by

messaging with the service processes

• Example: L4, Nucleus,

Taos, Mach, NT (hybrid)

µ−kernel entry User program Services call return

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Virtual Machine

"A running program is often referred to as a virtual machine - a machine that doesn't exist as a matter of actual physical reality. The virtual machine idea is itself one of the most elegant in the history of technology and is a crucial step in the evolution of ideas about software. To come up with it, scientists and technologists had to recognize that a computer running a program isn't merely a washer doing laundry. A washer is a washer whatever clothes you put inside, but when you put a new program in a computer, it becomes a new machine.... The virtual machine: A way of understanding software that frees us to think of software design as machine design." From David Gelernter's "Truth, Beauty, and the Virtual Machine," Discover Magazine, September 1997, p. 72.

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Virtual Machine

• Virtual machine monitor

– provide multiple virtual “real” hardware – run different OS codes

• Example

– IBM VM/370: Started in the 70’s. Check out – virtual 8086 mode – Java VM – VMware – Exokernel

Bare hardware Small kernel VM1 VMn . . . OS1 OSn user user

Exact copies of the bare hardware

Syscall trapped Privileged instructions trapped

Virtual Kernel Mode Kernel Mode User Mode Virtual User Mode

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Virtual 8086

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Java VM

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Hardware Support

• What is the minimal support?
• 2 modes
• Exception and interrupt trapping
• Can virtual machine be protected without such

support?

• Yes, emulation instead of executing on real machine

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Pro et Contra

Monolithic Layered VM C/S Micro kernel

• Many virtual

computers with different OS’es

• Test of new OS

while production work continues

• All in all:

flexibility

• Performance

issues?

• Complexity

issues?

• Performance
• More

unstructured

• Performance

issues?

• Clean, less bugs
• Clear division of

labour

• More flexible
• Small means less

bugs+manageable

• Distributed systems
• Failure isolation of

services at Kernel Level

• Flexibility issues?
• Performance issues?
• Clear division of

labour

• Performance

issues?

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“Truths” on Micro Kernel Flexibility and Performance

• A micro kernel restricts application level flexibility.
• Switching overhead kernel-user mode is inherently expensive.
• Switching address-spaces is costly.
• IPC is expensive.
• Micro kernel architectures lead to memory system degradation.
• Kernel should be portable (on top of a small hardware-

dependent layer).

Taken from J. Liedtke, SOSP 15 paper: ”On micro kernel construction” NO: Can be <50 cycles NO: 6-20 microsec round-trip, 53-500 cycles/IPC one way

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Concurrency and Process

• Problem to solve

– A shared CPU, many I/O devices and lots of interrupts – Users feel they have machine to themselves

• Answer

– Decompose hard problems into simple ones – Deal with one at a time – Process is such a unit

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Flow of Execution

Kernel Mode User Mode “Input finished” interrupt P1: Input syscall P1: CPU bound P2: CPU bound Trap handling; Scheduler; Dispatch; Trap handling; Scheduler; Dispatch; Trap handling; Scheduler; Dispatch; Int0x80 Timer 10-100ms (Could have started another process than P1) (Assume R to disk => long wait 10- 100’s ms)

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Procedure, Co-routine, Thread, Process

• Procedure, Function, (Sub)Routine
• Call-execute all-return nesting
• Co-routine
• Call-resumes-return
• Thread (more later)
• Process

– Single threaded – Multi threaded

User level non preemptive “scheduler” in user code

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Procedure and Co-routine

Call A; Call B; Call B; 1 1 2 2 Main A B Call A; Call B; Resume A; Resume B; Main A B Resume A; Return Return Never executed “User Yield when finished” “User Yield during execution to share CPU” Return

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Process

• Sequential execution of operations

– No concurrency inside a (single threaded) process – Everything happens sequentially

• Process state

– Registers – Stack(s) – Main memory – Files in UNIX – Communication ports – Other resources

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Program and Process

main() { ... foo() ... } foo() { ... }

Program

main() { ... foo() ... } foo() { ... }

Process heap stack

main foo

registers PC

Resources:

• comm. ports,

files, semaphores

PID

For at least one thread of execution The context

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Process vs. Program

• Process > program

– Program is just part of process state – Example: many users can run the same program

• Process < program

– A program can invoke more than one process – Example: Fork off processes to lookup webster

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Process State Transitions

Running Blocked Ready S c h e d u l e r d i s p a t c h Wait for resource Resource becomes available Create a process terminate P4 P3 P2 P1

P2 P1 ReadyQueue P4 P3 BlockedQueue

Scheduler Dispatcher Trap Handler Service

Current

Trap Return Handler U s e r L e v e l P r o c e s s e s

KERNEL MULTIPROGRAMMING

• Uniprocessor: Interleaving

(“pseudoparallelism”)

• Multiprocessor: Overlapping (“true

paralellism”)

PCB’s Memory resident part

Instruction Pointer (program counter) in the EIP register

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Process State Transition

Running Blocked Ready S c h e d u l e r d i s p a t c h W a i t f

• r

r e s

• u

r c e Resource becomes available Create a process terminate

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Process Control Block (Process Table)

• What

– Process management info

• State (ready, running, blocked)
• Registers, PSW, parents, etc

– Memory management info

• Segments, page table, stats, etc

– I/O and file management

• Communication ports, directories, file descriptors, etc.

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Discussion: What needs to be saved and restored

• n a context switch?
• Volatile state
• Program counter (Program Counter (PC) also called Instruction

Pointer (Intel: EIP))

• Processor status register
• Other register contents
• User and kernel stack pointers
• A pointer to the address space in which the process runs
• the process’s page table directory

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…and how?

• Save(volatile machine state, current process);
• Load(another process’s saved volatile state);
• Start(new process);

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Threads and Processes

Process Threads Kernel threads Kernel Address Space Kernel Level User Level

Project OpSys

• Trad. Threads

Processes in individual address spaces